The invention relates to a method of producing a computed digital image of a scene, which can be illuminated with different luminous intensities at a first illumination angle by a first light source. A plurality of individual images of the scene under different lighting conditions are taken by an image sensor and are offset against one another in a pixel-by-pixel manner in a data processing device for generating a resultant image.
The invention also relates to a device for producing a computed digital image, including a first light source, by which a scene to be recorded can be illuminated at a first illumination angle with different luminous intensities; an image sensor for the imaging detection of the illuminated scene and the output of digital individual images of the scene consisting of a plurality of pixels; control devices for the coordinated controlling of the light source and of the image sensor, so that individual images of the scene can be recorded under different lighting conditions; and a data processing device for the pixel-by-pixel offsetting of at least some of the individual images against one another.
Such methods and devices are known, for example, from applications for the image-supported monitoring or control of many different processes, in which an automatic analysis of a scene is desired and for whose simplification an image preprocessing is carried out which has the purpose of eliminating certain disturbance effects.
From German Patent document DE 100 62 977 A1, for example, a method and a device are known for monitoring the interior of a motor vehicle, in which case an infrared (IR)-sensitive CMOS digital camera takes individual images of the scene, specifically of the vehicle interior. Further, an IR radiation source for illuminating the scene is provided in the motor vehicle interior. For producing a result image, two individual images, respectively, are taken in rapid chronological succession. During the taking of the first individual image, the scene is illuminated by the IR radiation source at full power. During the talking of the second individual image, the scene is illuminated by the IR radiation source at a reduced power or, in an extreme case, is not illuminated at all. For producing the result image, the two individual images are then subtracted from one another pixel-by-pixel, so that the pixel values of the result image are corrected by the fraction of the background radiation, the so-called offset, which is independent of the IR radiation source.
As an object of monitoring the interior of a motor vehicle, a protection against an unauthorized access is known, for example, from the above-mentioned document. As another object, the monitoring of the positions of persons situated in the vehicle is known, for example, from German Patent document DE 199 08 167 A1. This dynamic type of monitoring is used, for example, for controlling passive safety systems, such as air bags. Particularly in the case of this type of monitoring systems, it is important that the objects of the scene which are of interest, such as the persons whose positions are to be detected, can be reliably distinguished from their own shadow or shadows which are cast by other objects of the scene. This task, which is simple for the human brain, presents considerable difficulties to automated systems which require extensive and therefore very time-intensive computing expenditures. Specifically, the time factor is extremely critical in the case of dynamic systems, and particularly passive seat belt systems.
It is therefore an object of the present invention to further develop a method of the above-mentioned type such that shadow effects of scene objects can be compensated in a simple manner.
It is another object of the present invention to further develop a device of the above-mentioned type such that it is suitable for implementing the method according to the invention.
The former object is achieved by providing a method of producing a computed digital image of a scene which can be illuminated with different luminous intensities at a first illumination angle by a first light source, a plurality of individual images of the scene under different lighting conditions being taken by an image sensor and being offset against one another in a pixel-by-pixel manner in a data processing device for generating a result image. At least another light source is provided by which the scene can be illuminated at an illumination angle with different luminous intensities which differs from the illumination angle of the first light source. In this case, the different illumination conditions when taking the individual images are achieved by different combinations of the luminous intensities of the first and of at least one other light source.
The effect of this method according to the invention consists of the fact that individual images of the same scene objects are produced but that the different illumination conditions provide that, during each taking of an individual image, the objects cast differently oriented shadows, so that in each individual image the values of different pixels are influenced by the casting of the shadow. It is thereby made possible that, within the scope of the subsequent offsetting of the individual images with one another, the influence of the casting of the shadow is compensated and an essentially shadow-free result image is obtained.
For simplifying the further description, the following definition is introduced. The value lj(n) in a pixel n in an individual image j can be represented in a simplified manner asIj(n)=α·ρ(n)·E(n)  (1)wherein p(n) is the surface reflectance of the scene point imaged in the pixel n; E(n) is the irradiation intensity fed to this scene point, and α is a proportional factor which describes influences, such as the light intensity of the used detector lens system, the efficiency of the detector and others and which, for reasons of simplicity, without limiting the generality, is assumed here to be equal to one (α=1) for all pixels.
It is true that different image processing methods are contemplated, in the case of which the pixels of the individual images influenced by the casting of shadows can be identified as such and their values during the production of the result image can be corrected for compensating the shadow effect. However, it is particularly advantageous for the offsetting of the individual images with one another to take place such that, while all pixel positions are treated the same, two pixels respectively of the result image, on which scene points illuminated by at least one light source are imaged, which are illuminated directly by different light sources or different numbers of light sources, have essentially the same pixel value with the exception of a factor representing the reflectance of the respective scene point.
As a result of the equal treatment of all pixel positions, that is, the application of the same mathematical operations to the entire image, time-consuming operations in the image itself, such as algorithms for detecting contours or the like, can be avoided. Such methods may be required, for example, within the scope of a subsequent analysis of the scene. However, for reasons of time, it is better to avoid them during the preparatory production of an initial image for the analysis, thus of the result image of the method according to the invention described here. As a result of the equal treatment of all their respective pixels, the individual images and possibly intermediate images computed therefrom in their entirety may be considered as variables of mathematical operations which can be applied, for example, by special image processing processors to all or at least a portion of the pixels in parallel. A significant time advantage can be achieved thereby in comparison to interpreting methods which have the actual image content as their object.
The operations to be applied to the images are to be selected such that each value of a pixel n of the result image essentially represents the reflectance p(n) of the surface of the scene point imaged in the pixel n. This means that scene areas, which are influenced by shadow effects, that is, are not directly illuminated by all light sources, by means of the image processing according to the invention, are adapted to one another and with respect to shadow-free scene areas. In other words, an ideal shadow-free illumination of the scene is simulated. Scene areas which are not illuminated by any light source, thus, can supply no relevant information, should preferably be ignored in this case, or should be set to a value which marks it as being ignorable during a subsequent further image processing. This marking should also preferably take place while all pixels are treated equally. In this context, “essentially” means that variations which are based on differences of the luminous intensity based on other reasons, for example, differences relating to the distance of the scene points from the light sources and/or the image sensor, are not taken into account.
Basically, a large number of suitable control modes of image taking and illumination are contemplated. However, a method was found to be particularly advantageous which is distinguished in that three individual images are taken for computing a result image, in which case two light sources are controlled such that, when a first individual image is taken, the first light source illuminates the scene with a first luminous intensity and the second light source illuminates the scene with a second luminous intensity differing from the first; during the taking of a second individual image, the first light source illuminates the scene with the second luminous intensity and the second light source illuminates the scene with the first luminous intensity; and, during the taking of a third individual image, both light sources each illuminate the scene with the second luminous intensity. As a result, the offsetting of the individual images with respect to one another becomes very easy. On the one hand, only a small number of individual images, specifically three, is required. On the other hand, as a result of the exchange according to the invention of the luminous intensities between the two light sources in the first and in the second individual image, essentially symmetrical illumination patterns are produced which facilitate the further processing of the data.
It is advantageous to preferably use such operations when offsetting the individual images with respect to one another, which can be implemented at low computing expenditures and do not lead to an excessive increase of the pixel noise. Thus, in particular, divisions by noise-loaded values should be avoided, if possible. On the other hand, it was found to be advantageous for the offsetting of the individual images with respect to one another to comprise the computing of the pixel-by-pixel difference and/or sum of the first and of the second individual image and/or the sum from the amount of the difference and of the sum of the first and of the second individual image. In addition, it was found to be advantageous to carry out a pixel-by-pixel subtraction of the third individual image.
An embodiment of the method according to the invention is particularly preferred in which, for computing the result image, a combination of mathematical operations is used which can be written according to the following formulaIout=|I1−I2|+(I1+I2)−2·I3  (2)wherein Iout is the value of a pixel of the result image; I1 is the value of the corresponding pixel of the first individual image; I2 is the value of the corresponding pixel of the second individual image; and I3 is the value of the corresponding pixel of the third individual image. Naturally, this formula is symbolic and is to be understood in such a manner that additive correction terms or correction factors which do not significantly change the image-related effect of the operation according to Equation (2) are also detected.
The above-mentioned second object is achieved by providing a device for producing a computed digital image, including a first light source by which a scene to be taken can be illuminated at a first illumination angle with different luminous intensities, an image sensor for imaging detection of the illuminated scene and the output of digital images of the scene consisting of a plurality of pixels, control devices for the coordinated controlling of the light source and of the image sensor, so that individual images of the scene can be taken under different illumination conditions, a data processing device for the pixel-by-pixel offsetting of at least some of the individual images with one another. At least one additional light source is provided by which the scene can be illuminated at a second illumination angle with different luminous intensities which differs from the first illumination angle. The light sources can be controlled such by the control devices such that the different illumination conditions during the taking of the individual images can be achieved by different combinations of the luminous intensities of the first and of at least one additional light source. This is the basic device-related prerequisite for being able to implement the above-described method. The prerequisite for implementing the above-described method also in the above-described preferred embodiments is the correspondingly suitable setting-up of the control devices as well as of the data processing system. The advantages achieved by the fact that the preferably program-related setting-up of the control devices and of the data processing system permits the implementation of the method according to the invention are illustrated by the above-described advantages of the method itself.
Although the method according to the invention is basically suitable for all types of illumination radiation, such as electromagnetic radiation in the optical, UV, IR, x-ray, microwave and radio wave range or acoustic radiation in arbitrary frequency ranges, particularly in the ultrasound range, the application in the close IR region was found to be particularly reliable and cost-effective. Particularly for applications in the environment of persons, this frequency range is particularly suitable because of its invisibility to the human eye, its relative harmlessness in the case of intensities which can be handled and its simultaneously still good spatial resolution because of the still short wavelengths.
A particularly advantageous field of application of the method according to the invention and of the device according to the invention is the monitoring of an interior of a motor vehicle; be it, for example, for the theft protection or for the detection of occupant positions, for example, for controlling passive safety systems.
As mentioned above, the above explanations neglect the influence of the distance of the scene points from the light sources. This is justified in many cases, particularly when no distance between a scene point and a light source is small in comparison to a distance between two scene points. In other words, that the light sources are arranged to be essentially at an equal distance from all scene points. In this case, the approximation implicitly made above that the luminous intensity E(n) for all directly illuminated scene points depends in the same manner on the radiation intensity of the respective light sources is correct. In addition, it is sufficient for many applications that, although the shadow effects are not completely compensated, they are significantly reduced.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.